Beds of mixed cationic exchange and anionic exchange resins are used in many industrial areas. One particular use of such resin beds is in deionization of water used in generating steam, wherein the water used should be substantially free from dissolved ionic materials so that a build-up of those previously dissolved salts may be avoided in the boiler and associated parts. Thus, water going into the boiler is first passed through the mixed resin bed to exchange cations and anions for protons and hydroxide ions, respectively.
A mixed resin bed has a finite exchange lifetime that is related to the exchange capacities of the resin, the amount of water passing through the bed and the ionic content of the water. After the exchange capacity of the resin bed is exhausted, the resin must either be regenerated or discarded.
An exhausted mixed resin bed is rarely discarded due to the relatively high cost of the resins. Rather, the exhausted mixed resin bed is regenerated to provide cation and anion exchange resins having substantially their original exchange capacity.
Several methods have been used to regenerate the exhausted resins in the bed. Typically, the resins are separate. Then, the exhausted cation exchange resin is treated with a protic acid so that protons replace the exchanged cations present in the exhausted resin, while the exhausted anion exchange resin is treated with a solution containing hydroxide ions to replace the anions. A method useful for carrying out the regeneration of exhausted resins is disclosed in co-pending, co-assigned U.S. patent application Ser. No. 493,828 filed concurrently herewith and now abandoned.
One general method for separating the cation exhange resin from the anion exchange resin utilizes resins having different densities, and suspends those resins in a classifying fluid such that they classify into layers in accordance with their differing densities. A problem with this technique has been that the densities of the different resins are not substantially different enough to afford a substantially complete separation of one resin from the other. To alleviate this problem, a quantity of substantially inert, particulate material having a density intermediate between that of the cation exchange resin and the anion exchange resin has been used.
Problems still exist in the separation of the resins even when an inert material having an intermediate density between that of the two exchange resins is used. These problems are associated with the determination of the location of the interface between the inert material and the cationic exchange resin, which is usually the heaviest, as well as the interface between the inert material and the anion exchange resin, which is usually the least dense.
Two methods of detecting the interface between the exchange resin and the inert material based on the conductivity properties of slurries of the material in a classifying fluid are those of U.S. Pat. Nos. 4,264,493 and 4,298,696. However, neither method is completely successful in automated systems.
In U.S. Pat. No. 4,264,439, a conductivity sensor is used to continuously measure the conductivity of a classified resin slurry as it passes from a holding tank. The sensor is connected to a recorder which exhibits a distinct change in the conductivity of the slurry to indicate the interface. The valve permitting passage of the slurry out of the holding tank is closed to retain the anion exchange resin within the holding tank when the distinct change is recorded.
There is, however, no discussion in U.S. Pat. No. 4,264,439 of what change the patentee relies upon, other than a drop, for determining the locations of the interface between the exchange resin and the inert material. An operator apparently must be taught the type of change to look for, be required to watch the recorder, detect that the required change in the conductance value has occurred and then subsequently shut the valve stopping passage of the slurry.
In U.S. Pat. No. 4,298,696, a conductivity sensor is used to determine the transition between the resins by detecting a decrease in the conductivity of the slurry as it passes out of its holding tank. A decrease to a set conductance value is implied for one embodiment, thereby making the use of classifying waters having substantially different conductance values identifying the interface between separations difficult. An alternative embodiment for use with classifying waters having variable conductivity utilizes two conductivity sensors at separate points to detect a predetermined difference in the conductivity. Again, however, there is no teaching of how to use the data gathered from such a sensor. There is also no teaching stating that the change in measured conductance can be used to automatically stop the flow of the resin slurry.
Each of the above methods therefore either requires the presence of an operator to make the detection of the change in conductivity which occurs when the interface between the exchange resin and inert material passes the sensor, or the use of the classifying fluid which itself has a relatively constant conductivity so that the preset value of conductance can be used by a machine or an operator for selecting the position of the interface.
It would therefore be beneficial if neither an operator nor a classifying fluid of relatively constant conductivity need be used for selecting the location of the interface between the exchange resin and inert material. The presently described invention provides such a benefit.